But VocalIQ's software — designed to help computers and people speak to each other in a more natural dialogue — hasn't found its way into Siri, according to an artificial intelligence (AI) expert with knowledge of voice recognition platforms.

"I think it's extremely unlikely that [VocalIQ] is in Siri now," the source said. "In fact, I'm pretty sure it isn't. But obviously Apple have got it for a reason."

The source said they could tell that VocalIQ's technology hadn't been integrated into Siri just by talking to the personal assistant. VocalIQ has a "a very particular style of dialogue ... and I haven't seen that in Siri," they said.

VocalIQ's technology was spun out of the University of Cambridge so it's "very technical, mathematical stuff," the source added. "It also takes a long time to get technology from a startup into a production product," they said.

But one of VocalIQ's early investors told Business Insider that it was "hard to tell" whether VocalIQ was in Siri or not. Earlier this year Steve Kovach reported:

"According to a source familiar with VocalIQ's product, it's much more robust and capable than Siri's biggest competitors like Google Now, Amazon's Alexa, and Microsoft's Cortana. In fact, it was so impressive that Apple bought VocalIQ before the company could finish and release its smartphone app. After the acquisition, Apple kept most of the VocalIQ team and let them work out of their Cambridge office and integrate the product into Siri."

He went on to write:

"It's still unclear when Apple plans to implement more of VocalIQ's capabilities into Siri. One source speculated that it may happen slowly over time, so as not to throw off users with a radical change. But it sounds like Apple is arming itself for a significant shift in how Siri works."